In recent years, porous spherical particles of a crosslinking polymer, particularly porous spherical particles having an average particle size of about 1 to 200 μm, have been widely used in the fields of toner, degloss for paint, gap adjusting material, filler for liquid chromatography, carrier for solid phase synthesis and the like. For these applications, porous spherical particles having a desired average particle size in the range of 1 to 200 μm, free of agglomerates particles and large particles, and having a narrow particle size distribution are extremely useful.
As a production method of porous spherical particles, seed polymerization, suspension polymerization and distribution polymerization have been conventionally used.
According to seed emulsion polymerization, particles having a narrow particle size distribution, i.e., monodisperse particles, can be obtained. However, to obtain about 5 μm particles, seed polymerization needs to be repeated plural times, and this method is not practical in view of many steps it requires. In practice, industrial production of particles of not less than 10 μm is difficult. According to a two-step swelling method, which is a modification of seed polymerization, particles of not less than 10 μm can be obtained. However, since it requires a swelling auxiliary agent, the swelling auxiliary agent needs to be removed after polymerization by washing, extraction and the like, the swelling operation itself requires time and thus, the method is poor in producibility. Moreover, particles having a uniform porous structure are difficult to obtain and reproducibility is hardly expected.
In the case of distribution polymerization, particles having an average particle size of 2 to 20 μm and a narrow particle size distribution can be obtained. However, due to the limitation on mechanism in that the particles are produced by the precipitation of polymer with the progress of polymerization, the polymerization system free of deformation of particle shape, variation of average particle size and extended range of particle size distribution is limited. Since the degree of crosslinking of the spherical particles produced by such a system is difficult, this method is not suitable for use requiring solvent resistance. While spherical particles of crosslinking polymer having a particle size of 0.5 to 5 μm can be produced by distribution polymerization of styrene-divinylbenzene in the presence of a high concentration distribution stabilizer, the resulting particles are non-porous.
On the other hand, production of porous spherical particles by suspension polymerization generally comprises mixing a monomer with what is called a porogen (i.e., organic solvent not involved in polymerization reaction and hardly soluble in aqueous medium, which dissolves monomer but does not dissolves polymer thereof), and stirring the mixture in an aqueous medium and the like. When a droplet containing a monomer is made small to a desired size in advance by stirring etc., spherical particles having a desired average particle size can be easily and economically produced. However, suspension polymerization affords particles having a wider particle size distribution, and agglomeration of particles and large particles tend to occur. Particularly, when porous spherical particles are produced by suspension polymerization, since a porous structure is constructed and the specific surface area rapidly expands with the progress of the polymerization, distribution stability of particles in an aqueous medium is markedly degraded, which easily produces agglomerates as compared to production of non-porous spherical particles. The presence of agglomerates particles and large particles, particularly amorphous agglomerates, causes rough surface, inconsistent design and the like, when used as a degloss for paint, and causes inconsistent filling property and inconsistent or degraded synthesizability when used for a carrier for solid phase. Therefore, agglomerates particles and large particles need to be removed from particles obtained by polymerization reaction, thus necessitating various labors for classification and the like.
There have been proposed various methods for solving the above-mentioned problems of suspension polymerization. For example, JP-A-52-47881, JP-A-52-47882 and JP-A-52-51483 disclose methods of narrowing the particle size distribution of the obtained polymer particles by using a nonionic surfactant as a suspension auxiliary agent during suspension polymerization of a vinyl monomer in the presence of a hardly soluble phosphate in an aqueous medium, and adding an anionic surfactant, a hardly soluble phosphate or a water-soluble polymer protective colloid during the polymerization. Moreover, JP-A-3-64308 discloses a method of narrowing the particle size distribution of the obtained polymer particles by adding an anionic surfactant and a hardly water-soluble organic salt during suspension polymerization of a styrene-based monomer in an aqueous medium containing a hardly water-soluble organic salt and a neutral water-soluble organic salt. However, since these methods use a non-ionic surfactant or a water-soluble organic salt as a suspension auxiliary agent, it is difficult to produce particles having a size of not more than 100 μm. In addition, these methods require addition of a distribution stabilizer such as anionic surfactant etc. during polymerization, besides addition of a distribution stabilizer such as a hardly water-soluble organic salt etc. in the initial stage of reaction, which means a step for the addition of several kinds of distribution stabilizers in two phases. Moreover, since these methods use a hardly water-soluble organic salt, solubilization and removal of the hardly water-soluble organic salt by an acid treatment etc. after polymerization become necessary, thus making the steps complicated. Furthermore, Japanese Patent No. 3467399 discloses a method of obtaining spherical microparticles free of agglomerate particles and large particles, which comprises adding an anionic surfactant during suspension polymerization of a vinyl monomer and a vinyl crosslinking agent in an aqueous medium containing a hardly water-soluble organic salt and an anion surfactant. This method certainly produces particles having a size of not more than 100 μm, but this method also requires addition of distribution stabilizer in two steps as mentioned above. In addition, since this method uses a hardly water-soluble inorganic salt solubilization and removal thereof is necessary, thus making the steps complicated.